This is the "An Angel appears to Hagar and Ishmael in the Desert" by Salvator Rosa. Painted in the Italian north, it's a beautiful representation of a scene from the Old Testament Book of Genesis. The story goes that Abraham was asked to banish his son Ishmael into the desert with little food or water. This painting shows the moment an angel appears to Hagar and Ishmael, pointing towards her sleeping child and showing them water.
Notice the silvery light illuminating the central tree, which has splintered branches behind which the angel appears. The angel is speaking to a troubled Hagar and indicating where they can find water. It's a powerful scene that highlights the importance of protection and provision in times of need.
An Angel appears to Hagar and Ishmael in the Desert hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.